Phosphorus is a minor, but essential, constituent of cells produced during growth in biological systems. When living organisms are supplied with sources of energy, essential chemicals, and other appropriate environmental conditions, they extract small amounts of phosphorus from the water for use in their reproduction and metabolism. Subsequent removal of cells reduces amounts of phosphorus in the water and, therefore, provides a mechanism for reducing its nutrient content.
Some nutrient removal occurs in all biological wastewater treatment plants. The organisms use chemicals in the wastewater to satisfy their metabolic needs and to produce new growth. Because phosphorus is an essential constituent of that growth, some phosphorus is accumulated in solids that are removed from the system for separate treatment and disposal. Improved efficiency in biological removal of phosphorus requires that the processes be modified to enhance nutrient uptake by the organisms, or to improve removals through other mechanisms.
The efficiency of phosphorus removal in conventional biological wastewater treatment usually is poor, perhaps fifteen to thirty percent, because organic loadings are too low to produce enough growth to take up more phosphorus. Biological phosphorus removal (BPR) processes are based on creating conditions that enhance the amount of phosphorus taken u and stored by organisms, followed by their removal for separate disposal.
While all organisms use some phosphorus in their metabolism, some are capable, in the presence of dissolved oxygen or nitrates, of taking up and storing excess phosphorus as complex polyphosphates. Other organisms commonly found in activated sludge systems do not have this capability. The basic approach in designing and operating BPR systems is to create conditions that favor growth of the phosphorus-accumulating types of organisms. When they are removed from the system during sludge wasting, their higher phosphorus content enhances removal of the nutrient.
Under anaerobic conditions, the desired organisms can assimilate simple organic compounds, such as acetates, and satisfy their energy needs by decomposing stored polyphosphates and releasing soluble orthophosphates. According to a typical generic BPR process, return sludge and incoming wastewater are mixed without adding oxygen, producing "anaerobic" conditions, which is the absence of both oxygen and nitrates. Organisms that contain stored polyphosphates adsorb organics from the liquid and begin to assimilate them releasing orthophosphates. Other types of organisms in the system are handicapped here because they need oxygen or nitrates in their metabolic reactions. This gives the phosphorus-storing organisms the first opportunity to use the food and that advantage increases their population share in the system. In the jargon of biological treatment, this "selects" organisms that can remove more phosphorus.
Phosphorus concentration in the wastewater usually increases drastically because of the phosphorus release during the anaerobic phase of this process, to a level that may be several times higher than that in the incoming flow. Actual phosphorus removal occurs in the next step, where the wastewater and organisms are mixed and aerated, as in a conventional activated sludge process. In the presence of oxygen, the organisms take up and store the phosphorus that was released in the anaerobic phase, in addition to much or all of that which entered with the wastewater. Subsequent settling removes phosphorus from the wastewater as a sludge in which the organisms usually contain about four to six percent phosphorus, instead of the typical two to three percent in conventional activated sludge.
Return activated sludge from the settling tank is pumped back to the beginning of the process to provide a high population of organisms for treating the incoming wastewater, as in the conventional activated sludge process. This generic process is deceptively simple in appearance. Actually, it is affected by many variables, some of which are not thoroughly understood. However, it is known that the anaerobic step is absolutely critical to successful biological phosphorus removal. Its main function is to give an advantage to the types of organisms desired in the system but that occurs only if both dissolved oxygen and nitrates are absent during the period. Presence of either can interfere severely with BPR because the desired organisms could lose their advantage and might not be "selected" in adequate numbers.
Production and maintenance of anaerobic conditions require enough organics to generate sufficient oxygen demand to expend both dissolved oxygen and nitrates. Accordingly, BOD of the wastewater is important, with successful performance being favored by strong wastes. For that reason, some investigators stress the importance of having a sufficiently high BOD/P ratio in the wastewater being treated. However, other factors involved in the process make it infeasible to identify a single specific ratio that could assure success or failure in biological phosphorus removal.
Many systems have been proposed for utilizing the above phenomena to remove nutrients from municipal wastewater. The well-known A/0 and A.sup.2 /0 processes developed by Air Products and Chemicals, Inc. (USA) are mainstream BPR processes that expose mixed return activated sludge and wastewater to anaerobic conditions for BPR organism selection prior to discharge into the mainstream aerobic zone. The A/0 and A.sup.2 /0 processes lack desirable control over the BPR selection process because the anaerobic zone is directly in the mainstream where plant influent flow rates and conditions vary considerably. The A/0 and A.sup.2 /0 processes often may require chemical precipitation of phosphorus in order to bring phosphorus levels in the effluent to an acceptable level.
The Phostrip process of Biospherics, Inc. (USA) accomplishes the biological removal of phosphorus in the mainstream of the plant by passing a portion of the return activated sludge through an anaerobic "stripping tank." Sludge solids from the stripper are returned to the mainstream aerobic zone where they provide organisms for treating the mainstream. A liquid (supernatant) separated from the solids in the stripper is treated with lime to form a phosphate precipitate that is removed in a settler. Thus, in the Phostrip process biological phosphorus removal is supplemented by a sidestream chemical precipitating process.
More recently, a sidestream system for selection of the desirable BPR organisms was reported in "Pilot Studies of Biological Phosphorus Removal," James C. Lamb, III, et al., 1986. This so-called "UNC Process" sidestreams the entire return activated sludge flow to an anaerobic zone where desirable BPR organisms are selected. A fermenter serves to ferment primary sludge to supply the food used in the BPR selection process. Variations on the UNC process are described in U.S. Pat. Nos. 4,874,519; 4,999,111 and 5,022,993 issued to Orange Water & Sewer Authority of Carrboro, N.C.
While many processes have been proposed to utilize the phenomenon of uptake of phosphorus by BPR organisms, there is still a need to further improve the "selection" process for desirable BPR organisms so that a municipal wastewater treatment plant can operate within its wide range of influent flow and characteristics, while reliably removing phosphorus in the wastewater to desirable levels well below 1 mg/l.